Outlook 2010 (64-bit) is my default mail-client (File > Options> General > bottom line; translated from Dutch, actual path could be slightly different in English). However, other programs complain there is no default mail-client selected. Adobe Photoshop Elements 8 refuses to send photos through Outlook because Outlook is not the default mail-client. Other programs complain as well and also refuse to start Outlook.

This must be a well-known problem, probably related to the 64-bit version of Outlook 2010. Is someone working on a solution?

For Microsoft Outlook, Win7 states: This program has all its defaults.

I even dug into Set program access and computer defaults and under Choose a default e-mail program I changed Use my current e-mail program to Microsoft Outlook.

Programs such as Photoshop refuse to send anything to Outlook and specifically ask to open Outlook and set it as default client. I think I read a couple of months ago that it definitely has something to do with the 64-bit version of Outlook. I assumed a patch would solve this problem. Two months later I'm still hoping...

About using 64-bit (don't mind you asking, but am not in the mood for deeper thoughts on this subject): I had no reason to stick to 32-bit. No special 32-bit add-ons or so.

Seems that the issue is, indeed, related to using the 64 bit version of Outlook: Check this thread in MS support forums.
You will see that some explanations are provided for what you are experiencing and the recommendation seems clear: use only the 64 bit version of Office if you absolutely must. If this is a big problem for you, I would suggest you uninstall Office 2010 x64 and install the 32 bit version instead. My previous experience with removing the 64 bit beta and adding the 32 bit release version suggest that you won't lose anything (configured email accounts, etc) and everything will just work after you complete the 32 bit install. A backup is always recommended, anyway, before doing something like this.

I am sorry this is the solution. I also ran a 64 bit beta of Office 2010, but given the "vehemence" of Microsoft's recommendation regarding the 32 vs 64 bit versions, I installed the 32 bit version once the release version was out.[/s]